Redox Thermodynamics of High-Spin and Low-Spin Forms of Chlorite Dismutases with Diverse Subunit and Oligomeric Structures

Chlorite dismutases (Clds) are heme b-containing oxidoreductases that convert chlorite to chloride and dioxygen. In this work, the thermodynamics of the one-electron reduction of the ferric high-spin forms and of the six-coodinate low-spin cyanide adducts of the enzymes from Nitrobacter winogradskyi (NwCld) and Candidatus nitrospira defluvii (NdCld) were determined through spectroelectrochemical experiments These proteins belong to two phylogenetically separated lineages that differ subunit (21.5 and 26 kDa, respectively) and, oligomeric dimeric and pentameric, respectively) structure but exhibit similar chlorite degradation activity. The E-o' values for free and Cyanide-bound proteins :were, determined to be -119. and, -397 mV for NwCld and -113 and -404 mV for NdCld, respectively (pH 7.0, 25 degrees C). Variable-temperature,spectroelectrochemical experiments revealed that the oxidized state of both proteins is enthalpically stabilized. Molecular dynamics simulations suggest that changes in the protein structure are negligible, whereas solvent reorganization is mainly responsible for the increase in entropy during the redox reaction. Obtained data are discussed with respect to the known structures of the two Clds and the proposed, reaction mechanism.